1. Field of the Invention
The present invention relates to a method of charging secondary batteries.
2. Description of the Prior Art
As a method of conventional charging a secondary battery, as shown in FIG. 1 (a), in general there is used a charging value control in which a battery has a peak point A in a end of charging completion during a charging, whereby a time point B is reduced a predetermined voltage.
It is known that, during the secondary battery charging, there occurs a peak voltage point A in a charge voltage curve as shown in FIG. 1 (a) at the end of the charging operation. According to the conventional charging method, in order to control the charging value it is generally known to use the peak point A or to detect other time point B where the battery voltage drops from the peak voltage A.
It is known to detect the voltage of the secondary battery during charging for controlling the charging current when the change of the battery voltage exceeds a predetermined value (Japanese patent publication 53-40695).
However, it is very difficult to detect accurately whether the voltage reaches the peak value by detecting only the voltage change and to avoid a detection error because the voltage change per unit time near the peak point A of the charge voltage and such small voltage change occurs not only near the peak point A but other various places.
The second method of detecting the time point B when a voltage .DELTA.V is reached, has an advantage that it is easier to detect the timing of the current control than the first mentioned method, but disadvantage is that the charged value changes battery by battery because the time point B fluctuates for the fixed voltage V due to the fact that the charging characteristic curve greatly change by the amount of the charging current and room temperature. In particular, since in the recently developed nickel hydrogen battery the voltage drop from peak point is small, so that the above mentioned problem still occurs.
In the third mentioned method, in which the gradient of the charging voltage curve is detected, the setting value is settable easier than the previous methods, it is difficult to control the charging current optimum by only detecting the period of time when the voltage change exceeds a predetermined value.
A further problem occurs when a pair of batteries are subjected to charging using any of the above mentioned charging methods. According to the conventional methods, if bad a battery is mixed or one or more batteries with relatively highly charged batteries are mixed, they tend to be overcharged, so that the cyclic charging property of the batteries may be deteriorated.